The invention refers to a process for the preparation of thiuram disulfides substituted with aliphatic, araliphatic and/or cycloaliphatic hydrocarbons from suitably substituted secondary amines and carbon disulfide in the presence of oxygen and a metalliferous catalyst.
Making use of known processes, thiuram disulfides can be obtained by the oxidative dimerization of salts of substituted dithiocarbamic acids. Hydrogen peroxide, nitrogen dioxide, chlorine, bromine, iodine, ozone, oxygen, sodium nitrite, sodium hypochlorite, sulfur chlorides, potassium perbromate, selenic acid, or ammonium persulfate are used as oxidant. Tetramethyl thiuram disulfide, one of the most important representatives of this category of compounds, is made on an industrial scale by means of a two-stage process. In the first stage, dimethylamine and carbon disulfide in aqueous sodium hydroxide are reacted to form sodium-N,N-dimethyl dithiocarbamate. In the second stage the dithiocarbamate is oxidized with hydrogen peroxide in the presence of sulfuric acid (Bios 1150, Fiat 1018), with chlorine (U.S. Pat. Nos. 2,751,514 and 2,751,416), or electrolytically (German patent application disclosure Nos. 28 02 260 and 28 03 591).
In the process of German Pat. No. 12 26 564, a secondary alkyl-, aryl- or alkylarylamine is reacted with carbon disulfide in an aqueous or non-aqueous medium and in the presence of an oxygen-containing gas and a metal catalyst to form substituted thiuram disulfide. A sulfonated or carboxylated metal phthalocyanine of the 8th group of the periodic system, as for example cobalt phthalocyanine, is used as catalyst. In this process, the yield is relatively low; at best, it is about 25% of theoretical. When aromatic amines such as diphenylamine are used, the process of German Pat. No. 12 26 564 does not result in the formation of thiuram disulfide. In addition, the preparation and industrial use of the cobalt catalyst are problematical.
The use of a metalliferous catalyst in the oxidation of alkali salts of substituted dithiocarbamic acids with oxygen is also known. According to the process of German published application No. 11 65 011, the oxidation is carried out in an aqueous solution of a sulfonated or carboxylated Group VIII metal phthalocyanine at a pH of about 7 to 12. However, materials used in this process add to its expense and form unusable byproducts. Lye is needed for the preparation of the dithiocarbamates and hydrochloric acid is required for pH adjustment, and these form unusable sodium chloride. Further, the industrial preparation and application of these Group VIII metalliferous catalysts is problematical.
The use of an ammonium salt of dithiocarbamic acid, instead of the alkali salts, is also already known. In the process of German patent application disclosure No. 25 27 898, ammoniumdimethyldithiocarbamate is oxidized by means of hydrogen peroxide in an aqueous solution of sulfuric acid at a pH of from 5 to 7 to yield a suspension of solid tetramethyl thiuram disulfide in an aqueous ammonium sulfate solution. After the solid tetramethyl thiuram disulfide has been filtered off the resulting filtrate must be concentrated down to the solubility limit of the ammonium sulfate, resulting in its precipitation. The ammonium sulfate could be used as a fertilizer, but only if the adhering dithiocarbamate is removed. This makes the ammonium sulfate an undesirable byproduct.
Surprisingly, it has now been found that upon oxidation of secondary amines with a pK.sub.a .gtoreq.8 and carbon disulfide with oxygen to form thiuram disulfides, the yield and selectivity can increase considerably when certain metals or derivatives are selected as catalyst.
An object of the invention is a process for the preparation of a thiuram disulfide substituted with aliphatic, araliphatic, and/or cycloaliphatic hydrocarbon radicals by reacting a suitably substituted secondary amine with carbon disulfide in a solvent and in the presence of oxygen or a gas containing oxygen and a metalliferous catalyst. The reaction is carried out with a secondary amine with a pK.sub.a value of .gtoreq.8, and at temperatures from 0.degree. to 200.degree. C., and comprises reacting carbon disulfide and the secondary amine in a molar ratio of 1.0 to 1.2:1 in the presence of oxygen or a gas containing oxygen and the metalliferous catalyst. A second embodiment comprises reacting carbon disulfide and the secondary amine in a molar ratio of 0.9 to 1.1:2.0 to 2.2, and then reacting the resulting reaction product with 1.0 to 1.2 moles of carbon disulfide per mole of carbon disulfide originally reacted, in the presence of oxygen or a gas containing oxygen and the metalliferous catalyst. A still further embodiment comprises reacting carbon disulfide and the secondary amine in a molar ratio of 0.9 to 1.1:2.0 to 2.2, reacting the formed dithiocarbamate with carbon disulfide in a molar ratio of 1.0:1.0 to 1.2 in the presence of oxygen or a gas containing oxygen and the metalliferous catalyst, wherein said catalyst is selected from the group of metals including cerium, manganese, copper, molybdenum, vanadium, a derivative of the said metals, or a mixture of said metals or derivatives.
The process pursuant to the invention is suitable for the preparation of many different substituted thiuram disulfides. When only a single secondary amine is used as reactant, one obtains a thiuram disulfide carrying the same substituents on both nitrogen atoms. In the case of a symmetrically substituted secondary amine, one therefore obtains a thiuram disulfide with four identical substituents. When two different secondary amines are used as reactants, one can, depending upon the process conditions such as differences in the basicity of the amines or molar ratios, obtain thiuram disulfides with two different substituted nitrogen atoms. Varying quantities of the two symmetrically substituted thiuram disulfides can thereby be formed as by-products.
The metalliferous catalysts used as cerium, manganese, copper, molybdenum or vanadium in elemental form, or as salts, complexes, or in the form of their organic compounds. Among these metals or their derivatives, copper, manganese and cerium have a greater catalytic effectiveness compared with molybdenum and vanadium, but the two latter metals and their derivatives are also excellent oxidation catalysts.
Elemental copper is preferably used as copper powder. Suitable copper compounds include all mono- or divalent inorganic, organic, simple, or complex copper salts. Examples of suitable monovalent copper salts are copper(I) chloride, bromide and iodide, addition compounds of these copper(I) halides with carbon monoxide, complex copper(I) salts, such as the alkali chlorocuprates, complex ammoniacates of copper(I) cyanide, such as cyanocuprates, e.g. potassium tricyanocuprate(I), double salts with copper(I) thiocyanate, copper(I) acetate, copper(I) sulfide and complex double sulfides or copper(I) sulfide and alkali polysulfides. Examples of suitable copper(II) salts are copper(II) chloride, bromide, sulfide, nitrite, thiocyanate, or cyanide, Cu(II) salts of carboxylic acids, such as copper(II) acetate, copper dithiocarbamate, as well as the complex ammoniacates of copper(II) salts. Copper(I) oxide is also very well suited as catalyst.
Suitable manganese-containing catalysts include manganese powder, manganese dioxide, potassium permanganate, manganese acetate, and the manganese dithiocarbamates as well as the other manganese derivatives corresponding to the above-mentioned copper compounds.
Suitable cerium catalysts include metallic cerium, cerium dioxide, cerium(II) chloride, cerium chloride, cerium chlorocomplex salts, cerium nitrate, cerium nitrato salts, cerium sulfate, cerium carbonate, cerium oxalate and the cerium sulfides.
Examples of suitable vanadium catalysts are the vanadium oxides, chlorides and sulfates, as well as the known double and complex salts.
Finally, the suitable molybdenum catalysts include molybdenum oxides, chlorides, sulfides and fluorides, the molybdates, and the known complex acido salts. Mixtures of several of the above-mentioned catalysts may also be used.
The required quantity of metalliferous catalyst is surprisingly small. Preferably, it is within a range of from 0.01 to 5 millimoles per mole of secondary amine. Smaller quantities of catalyst may also be used, but reaction times will then be extended. Larger quantities of catalysts should be avoided because of the danger that the catalyst would precipitate and contaminate the reaction product.
The following are examples of aliphatic secondary amines suitable for the process pursuant to the invention: dimethylamine, diethylamine, dipropylamine, diisopropylamine, dibutylamine, di-sec-butylamine, di-tert.-butylamine, di-(2-methylpropyl)-amine, dipentylamine, di-(1-methylbutyl)-amine, di-(2-methylbutyl)-amine, di-(3-methyl-butyl)-amine, di-(1,1-methylpropyl)-amine, di-(2,2-dimethyl-propyl)-amine, di(1,2-dimethylpropyl)-amine, dihexyl-amine, di-(1-methylpentyl)-amine, di-(2-methylpentyl)-amine, di(3-methylpentyl)-amine, di-(3-ethylpentyl)-amine, di-(1,1-dimethyl-butyl)-amine, di-(2,2-dimethylbutyl)-amine, di(3,3-dimethylbutyl)-amine, di-(2,3-dimethybutyl) amine, di-(1-ethylbutyl)amine, di-(2-ethybutyl)-amine, diheptylamine, di-(1-methylhexyl-amine, di-(2-methylhexyl)-amine, di-(3-methylhexyl)-amine, di-(4-methylhexyl)-amine, di-(5-methylhexyl)-amine, di-(1-ethylpentyl) amine, di-(2-ethylpentyl)-amine, di-(3-ethylpentyl)-amine, dioctylamine, di-(1-methylheptyl)-amine, di-(2-methylheptyl)-amine, di-(3-methylheptyl)-amine, di-(4-methylheptyl)-amine, di-(5-methylheptyl)-amine, di-(6-hexyl)-amine, di-(3-ethylhexyl)amine, di-(4-ethylhexyl)-amine, methylethylamine, ethylbutylamine, dilaurylamine, didodecylamine, ditridecylamine, dipalmitylamine, distearylamine and dioleylamine.
Suitable cycloaliphatic secondary amines include the following: dicyclohexylamine, 4,4'-dimethyldicyclohexylamine, 3,3'-dimethyldicyclohexylamine, 2,2'-dimethyldicyclohexylamine; N-methylcyclohexylamine, N-ethylcyclohexylamine, N-propylcyclohexylamine, N-isopropyl-cyclohexylamine, N-butyl-cyclohexylamine, N-sec.-butylcyclohexylamine, N-tert.-butyl-cyclohexylamine, N-pentycyclohexylamine, N-(1-methylbutyl)-cyclohexylamine, N-(2-methylbutyl)-cyclohexylamine, N-(3-methyl-butyl)-cyclohexylamine, N-(1,1-dimethylpropyl)-cyclohexylamine, N-(2,2-dimethylpropyl)-cyclohexylamine, N-(1,2-dimethylpropyl)-cyclohexylamine, N-hexylcyclohexylamine, N-(1-methylpropyl)-cyclohexylamine, N-(2-methylpentyl)-cyclohexylamine, N-(3-methyl-pentyl)-cyclohexylamine, N-(4-methylpentyl)-cyclohexylamine, N-(1,1-dimethylbutyl)-cyclohexylamine, N-(2,2-dimethylbutyl)cyclohexylamine, N-(3,3-dimethylbutyl)-cyclohexylamine, N-(2,3-dimethylbutyl)-cyclohexylamine, N-(1-ethylbutyl)-cyclohexylamine, N-(2-ethylbutyl)-cyclohexylamine, N-heptylcyclohexylamine, N-(1-methylhexyl)-cyclohexylamine, N-(2-methylhexyl)-cyclohexylamine, N-(3-methylhexyl)-cyclohexylamine, N-(1-ethylpentyl)-cyclohexylamine, N-(2-ethylpentyl)-cyclohexylamine, N-3-ethylpentyl)-cyclohexylamine, N-octylcyclohexylamine, N-(1-methylpentyl)-cyclohexylamine, N-(2-methylheptyl)-cyclohexylamine, N-(3-methylheptyl)-cyclohexylamine, N-(4-methylpentyl)-cyclohexylamine, N-(5-methylheptyl)-cyclohexylamine, N-(6-methylheptyl)-cyclohexylamine, N-(1-ethylhexyl)-cyclohexylamine, N-(2-ethylhexyl)-cyclohexylamine, N-(3-ethylhexyl)-cyclohexylamine, N-(4-ethylhexyl)cyclohexylamine, dicyclopentylamine, N-methylcyclopentylamine, N-ethylcyclohexyl-amine, N-methylcyclobutylamine, N-methylcycloheptylamine and N-ethylcycloheptylamine.
Suitable araliphatic secondary amines are the listed aliphatic and cycloaliphatic amines having one or several of their carbon chain hydrogen atoms substituted by aryl radicals, as for example the following: dibenzylamine, di-(2-phenylethyl)-amine, di-(2-phenylpropyl)-amine, di-(3-phenylpropyl)-amine, N-methylbenzylamine, N-ethylbenzylamine, and N-propylbenzylamine.
As mentioned hereinabove, the substituents of the secondary amine may be identical or different. They may also be connected with one another via a common bridge bond. Examples of such cyclic amines are piperidine, pyrrolidine and derivatives, as well as other nitrogen heterocycles.
In the process pursuant to the invention, the oxidant used in oxygen, or a gas containing oxygen such as air. Non-aqueous solvents suitable for the present process include aromatic hydrocarbons such as benzene, toluene, xylene, or nitrobenzene; aliphatic esters; alkyl ether; lower alcohols, such as the C.sub.1 -C.sub.4 alcohols including methanol, ethanol, isopropanol, n-propanol, t-butanol, and amyl alcohol; chlorinated hydrocarbons, such as dichloromethane, chloroform, dichloroethane, trichloroethane; and aprotic solvents such as dimethyl formamide, acetonitrile, dimethyl acetamide, dimethyl sulfoxide, and hexamethylphosphoric triamide. Suitable aqueous solvent include water/alcohol mixtures. High yields and selectivities may be obtained in pure water, but in general the reaction rate in water is slower than in the above-mentioned non-aqueous solvents. Preferred solvents include aromatic hydrocarbons, low alcohols, and alcohol/water mixtures.
The process pursuant to the invention is carried out at temperatures in the range from 0.degree. to 200.degree. C., preferably at 20.degree. to 90.degree. C. temperatures above 90.degree. C. are less preferable for economic and safety reasons. Preferably, the process pursuant to the invention is carried out at oxygen pressures, or at partial oxygen pressures of at least 0.1 bar. Expectably, the reaction rate increases with rising oxygen pressure.
In principle, the present process may be carried out according to three methods of synthesis. In the first process variant, a secondary amine and carbon disulfide at a molar ratio of 1.0 to 1.2:1, in the presence of oxygen or a gas containing oxygen and the metalliferous catalyst can be reacted directly to form the corresponding thiuram disulfide. In the second variant, the carbon disulfide and the secondary amine at a molar ratio of 0.9 to 1.1:2.0 to 2.1 may be reacted. Thereafter, the resulting reaction mixture is reacted with approximately 1.0-1.2 moles of carbon disulfide per mole of carbon disulfide reacted in the first step in the presence of the metalliferous catalyst and oxygen or .alpha. gas containing oxygen. In the third process variant, the dithiocarbamate formed by the reaction of the secondary amine and carbon difulfide is isolated as an intermediate product, after which this dithiocarbamate is reacted with 1.0-1.2 moles of carbon disulfide per mole of carbon disulfide reacted in the first step, in the presence of oxygen or a oxygen-containing gas and the metalliferous catalyst. The duration of the reaction depends upon the process conditions, and lies within a range from a few minutes to several hours. Under optimal temperature and oxygen pressure conditions it amounts to a few minutes.
The process pursuant to the invention can be carried out in a simple manner, as by forcing oxygen or the gas containing oxygen onto the reaction solution under the indicated pressure and temperature conditions, or by conducting it into or through the reaction solution. This reaction solution consists of solvent, secondary amine, catalyst and carbon disulfide, or of solvent, catalyst and dithiocarbamate, or of the reaction mixture obtained in the reaction of secondary amine and carbon disulfide in the solvent, and the catalyst. In most cases, as for example with tetramethyl thiuram disulfide, the end product will immediately precipitate out of the reaction mixture and can be filtered off. In other cases, one obtains the desired end product when the reaction mixture is cooled or concentrated. Liquid products may be separated by means of distillation or extraction.
In an industrial-scale process pursuant to the invention it is advantageous to circulate the mother liquor so as to preclude the need for steady addition of fresh metalliferous catalyst. It is, for example, possible to run more than ten reaction cycles with a constant high yield and without loss in catalyst activity. Practically quantitiative yields and selectivities can be obtained in the process pursuant to the invention. The resulting products are very pure and generally need no further purification. Compared with the known two-stage process in which the dithiocarbamate is synthesized first, the present single-stage process is advantageous in that it is economical and in that no auxiliary materials are used. In contrast to the single-stage process known from German published application No. 11 65 011, the present process utilizes simple and inexpensive catalysts. A further advantage is that in the industrial-scale process pursuant to the invention, soluble, recirculable catalysts which do not lose their activity and which result in practically quantitative yields are used.